Methods of fabricating micromechanical devices from semiconductor materials such as silicon are well known. One such method is described in Applicant's commonly assigned pending U.S. Pat. application Ser. No. 143,515 entitled: "Method and Apparatus for Semiconductor Chip Transducer" and incorporated herein by reference.
As disclosed in the above-referenced pending application, diffusion of a silicon substrate with selective etch resistant P-type boron dopant is utilized to form both a vibrationally supported planar element or similar resonant structure, as well as "flexures" or "bridges" supporting such structures to an adjacent frame, for vibration at a resonant frequency.
The Boron diffusion used to define the flexures and the subsequent etching of the surrounding silicon, however, causes shrinking that creates a high tensile forces or stress in and around the flexures. Such stress can be substantial and may be the cause of structural failure in the micromechanical device. Additionally, the high tensile force increases the resonant frequency of the structure, requires a higher drive voltage and induces of cross coupling to other adjacent structures or elements.
To date, attempts at reducing these tensile forces have included providing a tension relief beam within the resonant structure, and to which the flexures are attached. These tension relief beams are allowed to deflect, providing the necessary tension relief for the flexures and the resonant structure. Reducing the tensile forces in the resonant structure also achieves the result of lowering the resonant frequency of the structure. Accordingly, calculations must be made to arrive at an approximate length and width of the tension relief beams which will produce a resonant structure having approximately the desired resonant frequency. However, since many random fabrication variables may be involved, the selection of the tension relief beam length and width results in a mere coarse definition of the resonant frequency, and may not provide the degree of accuracy in the resonant frequency required for a particular design.